1. Field of the Invention
The present invention relates to an image capturing apparatus and a control method of an image capturing apparatus, particularly relates to a correction processing technique for image data obtained by capturing an image.
2. Description of the Related Art
An image capturing apparatus typified by a digital still camera and a video camera is equipped with an image sensing element which converts an optical image of an object into an electrical signal. Among various types of image sensing elements, especially a CMOS image sensor including an amplifier arranged on each column (to be referred to as a column amplifier hereinafter) is a mainstream image sensing element. However, this image sensor often generates variations in luminance value due to variations in electrical characteristics of the individual column amplifiers, and, in turn, generates vertical streaks. To correct these vertical streaks, a shading portion (to be referred to as a VOB portion hereinafter) is provided to cover the entire pixel region including all pixels aligned horizontally (in the row direction) at the end portion (on about 10 lines) of the pixel region, and correction data is generated for each column based on signals (to be referred to as VOB signals hereinafter) output from this VOB portion, thereby executing correction using the difference between the correction data and the image data in the effective pixel region.
VOB data contains random noise components, so VOB signals are averaged for each column to generate correction data. This averaging processing produces an effect of eliminating the random noise components, but requires a large number of VOB signals and, for example, requires a long period of time to process 256 data corresponding to about 25 frames until the correction data converges.
An electronic antivibration function is implemented by defining a region (extraction region) smaller than the entire pixel region of the sensor as an effective pixel region for use in image output, and a pixel region other than the effective pixel region as an electronic antivibration region, and variably setting the extraction region for each frame. This electronic antivibration function is implemented by reading out signals from a pixel region including the electronic antivibration region from the CMOS image sensor, performing the aforementioned vertical streak correction, and arbitrarily extracting output image data.
Since the CMOS image sensor can randomly access (read out signals from an arbitrary region on) the image data, the aforementioned electronic antivibration function can be implemented by variably controlling the readout region. The region (the number of pixels) from which the sensor reads out signals is limited in this way, thereby reducing the frequency and power consumption in pixel signal transfer.
However, in the aforementioned electronic antivibration technique, the readout region changes for each column frame. Hence, when the readout region shifts horizontally (in the row direction), correction is executed based on correction data for different columns because correction data is generated by averaging VOB signals.
For this reason, to obtain appropriate correction data, it is necessary to read out VOB signals from the same readout region by about 25 frames, so precise correction data is not generated in electronic antivibration control under which the readout region is variably set for each frame, thus generating vertical streaks.